This application claims the priority of German Patent Document 100 63 654.3, filed Dec. 20, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and an apparatus for producing current values dependent on the position of the accelerator pedal for the purpose of adjusting the power of one or more drives for moving a mobile device, which has a fuel cell for supplying energy to the drive or drives and to other electrical loads.
The prior art already includes a method for dynamic control of the power of an electric drive unit in a vehicle which is supplied with electrical energy by a fuel cell (DE 43 22 765 C1). The driving powers are associated with the positions of the accelerator pedal. The mass flow of air required to provide the desired driving power is calculated as a function of the position of the accelerator pedal and is supplied by a speed-controlled compressor arranged in the air outlet line of the fuel cell. The desired power value that is fed to the drive unit is corrected to ensure that the drive unit never demands more power than what is instantaneously being produced by the fuel cell. The drive unit can also exercise a limiting action on the power demand. In these known methods, dead times result in a relatively long response time when the vehicle is accelerated.
The prior art also includes a method for dynamic adjustment of the power of a fuel cell in a vehicle that has a drive unit supplied with energy by the fuel cell. A desired value for the power of the fuel cell is determined as a function of a demand at the accelerator pedal. Below a limiting value for the instantaneous voltage of the fuel cell, a desired current value for the fuel cell is determined from the instantaneous speed of the drive motor by means of a characteristic map for the current. Above a limiting value for the instantaneous voltage of the fuel cell, a desired power value is determined from the instantaneous speed of the drive motor by means of a power map for the drive motor, and this desired value is used to determine a desired current value for the fuel cell from the voltage of the fuel cell. The respective currents are increased by a predetermined amount as a function of the currents required by the other electrical units of the vehicle (DE 195 40 824 A1).
Fuel-cell system s that have compressors for adjusting the mass flow of air exhibit certain delays in their response. Delays also occur if the fuel gas for the fuel cell is supplied by a system for the production of hydrogen gas. The response time of the fuel-cell system in the vehicle must be taken into account when producing the desired current value. Releasing the respectively required desired current value too quickly leads to a collapse in the voltage of the fuel cell and hence to a malfunction. Excessively slow release of the desired current value associated with a particular position of the accelerator pedal does not exploit the potential response time.
The response time of the fuel-cell system depends on numerous factors. Some of these factors are the instantaneous load level, the type of current or power demand, i.e. the time characteristic of the demand, such as a jump or ramp, the rate of rise and the instantaneous condition of the fuel-cell system. The parameters that influence the response time here are numerous. At the time when the current is demanded, not all of them are known. Determining all the parameters is possible only with a high outlay.
To avoid malfunctions in the case of rapidly changing current demands, a fixed transfer characteristic can be set in one element of a control circuit, e.g. an xth-order delay can be provided.
One object of the invention is to provide a method for producing current values as a function of the position of the accelerator pedal for the purpose of adjusting the power of one or more drives for moving a mobile device.
Another object of the invention is to achieve an optimum dynamic response, without explicitly determining the parameters that influence the response of the fuel cell.
These and other objects and advantages are achieved by the method and apparatus according to the invention, which comprise the steps of measuring the output voltage of the fuel cell during a change in a desired current value as a function of the position of the accelerator pedal and monitoring the output voltage against a lower limiting voltage value, reducing the desired current value by the amount of current drawn by the other electrical loads and the current required to adjust the power of the fuel cell to the driving power corresponding to the desired current value and adjusting the remaining part of the desired current value, as the available current for the production of a corresponding desired torque value, in such away that the limiting voltage value is not undershot to a degree that would impair the operation of the drive or drives and the other current-consuming loads. In the adjustment of the available current value in relation to the limiting voltage value, an optimum, i.e. minimum, rise time of this current value is achieved without the fuel cell or drives and the other electrical loads of the fuel-cell system adopting an unwanted operating condition. In the case of a change in the position of the accelerator pedal, the dynamic response of the fuel-cell system (i.e. the fuel cell and the loads supplied) depends on the current demand of the fuel cell and of its auxiliary units and on the respective operating condition of the other electrical loads of the fuel-cell system. By means of the invention, an optimum dynamic response is achieved without the need explicitly to determine or model all the requirements and/or dependencies of the electrical loads in the respective operating condition. The requirements or dependencies are allowed for automatically by the invention, with the result that the outlay for setting and parameterizing the fuel-cell system is low.
The lower limiting voltage value is preferably the rated or operating voltage of the electrical loads supplied by the fuel cell. The electrical loads in the mobile device thus maintain the rated or operating voltage even during the transition of the fuel-cell system and the drives to a different operating condition. During the change in the desired current value, the value of the available current is sufficient to ensure that the loads in the fuel-cell system, including the drive or drives, can draw the currents required for them to operate satisfactorily. As a result, the drive or drives change to the new operating condition in as short a time as possible. It should be pointed out here that other means for adjusting the power of a drive, such as manually actuated members, are equivalent in mobile devices to the foot-actuated accelerator pedal.
In a preferred embodiment, the output voltage of the fuel cell is held at a value that is uncritical for the operation of the drive or drives and the fuel-cell system by adjusting the available current by closed-loop control, at least during the change in a desired current value. This method ensures that the output voltage of the fuel cell does not fall below a critical value to an extent that would be undesirable for the operation of the fuel-cell system, if at all. The available driving-current value is controlled in the optimum manner, with the result that the drive or drives make the transition to the new operating condition, e.g. a new speed or power, in as short a time as possible.
In an expedient embodiment, the available current value is adjusted in such a way during the change in the desired current value that the first or second derivative of the output voltage of the fuel cell with respect to the current value remains the same. With this method, it is possible to achieve a desired driving response when changes in the position of the accelerator pedal occur. By controlling the second derivative of the output voltage of the fuel cell in accordance with the available current value, for example, it is possible to achieve jerk-free adjustment of the power or speed during the transition to new speed or power values. It is advantageous here to provide a cascade control system, the control of the output voltage of the fuel cell taking place in an outer control circuit.
In an apparatus of the present invention, a control element is supplied with the deviation formed from the difference between the lower limiting voltage value and the output voltage of the fuel cell. A current value, proportional to the deviation and having a current-limiting characteristic, is superimposed by the controller on the available current to produce a correct current value that is available for conversion to a desired torque value. The available current, as discussed above, is the difference between the desired current value and the sum of the currents drawn by the auxiliary units of the fuel cell and the currents drawn by the other loads. With this controller, it is possible to achieve an optimum dynamic response from the fuel-cell system; i.e., the response time of the drive or drives after changes in the operating point is as short as possible. The lower limiting voltage value is, in particular, the rated or operating voltage of the electrical loads and of the drive. The word xe2x80x9cdrivexe2x80x9d should here be taken to mean the unit consisting of a power converter and a motor.
The controller has a proportional action. A current proportional to the negative deviation is therefore subtracted from the current available to form the torque of the drive. It is advantageous for many applications if the profile of this current value has a particular characteristic. Steady changes in power or speed may be desired, for example. In such a case, a controlled or secondary controlled variable can be the first derivative of the output voltage of the fuel cell with respect to the current. Jerk-free behaviour can be achieved by using the second derivative of the output voltage of the fuel cell with respect to the available current as controlled or secondary controlled variables.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.